Aviation and the Global Atmosphere

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6.3.3. Radiative Forcing for O3

Ozone is a potent greenhouse gas whose concentration is highly variable and controlled by atmospheric chemistry and dynamics. Aircraft emissions of NOx accelerate local photochemical production of O3 in the troposphere; modeling studies suggest that these emissions are responsible today for average O3 enhancements of 2-5 ppbv in the middle troposphere at northern mid-latitudes, where most aircraft fly (see Chapters 2 and 4). This ozone increase will generally be proportional to the amount of NOx emitted (Grewe et al., 1999), but evolving atmospheric composition, including increases in surface sources of combustion-related NOx, will affect the aircraft impact.

Four subsonic ozone perturbations based on detailed 3-D patterns of NOx emissions were chosen (along with their control atmospheres) for the calculation of RFs: NASA-1992, NASA-2015, FESGa (tech 1), and FESGe (tech 1) at 2050. Results for NASA-1992 were scaled by 1.15 to give NASA-1992*, and those for NASA-2015 by 1.05 to give NASA-2015*. Table 6-1 describes some of the basic properties of these aircraft scenarios, including total NOx emissions. Chapter 4 supplied the seasonal pattern of O3 perturbations for these scenarios based on model calculations and reported a factor of 2 uncertainty in this best value. The modeled RFs from these O3 perturbations agree quite well, and the stratospheric temperature adjustment does not greatly affect the result (as was confirmed by two independent model calculations). Given the predominantly tropospheric perturbation, the uncertainty in modeling RF is small, and the uncertainty in the final result may be a factor of only 3. The ozone RFs are +0.023 W m-2 for NASA-1992*, +0.040 W m-2 for NASA-2015*, and +0.060 W m-2 for FESGa (tech1) 2050 (see also Table 6-1).

The development of atmospheric chemistry models in the past 2 years has allowed a consensus to build such that aircraft ozone perturbations can be calculated with a likely (2/3 probability) range of about a factor of 2 (higher/lower, see Chapter 4). Our estimate of the resulting RF for this predominantly tropospheric perturbation does not significantly enhance that interval.

The NASA-1992, NASA-2015, and FESGa (tech 1) at 2050 scenarios produce global mean column ozone increases (predominantly tropospheric) of 0.5, 1.1, and 1.7 Dobson Units (DU), respectively. Our estimate of the increase in tropospheric ozone associated with all anthropogenic changes (IS92a including aircraft plus surface emissions of NOx, CO, and hydrocarbons) is about 3 DU from 1992 to 2015 and 7 DU from 1992 to 2050. These results represent an advance in our understanding since the Second Assessment Report (IPCC, 1996), when future ozone changes were scaled only to CH4 increases and did not include the effects of doubling NOx emissions from 1990 to 2050.

Two HSCT cases with detailed 3-D emission scenarios-one with 500 aircraft and the other with 1,000 aircraft-were used to calculate RF from stratospheric O3 and H2O perturbations (see Table 6-1). Most of the ozone change occurs above the tropopause; thus, there is poorer agreement among RF models and a greater difference in RF values after stratospheric temperatures adjust. The ozone perturbation calculated for 500 supersonic aircraft with the 2015 background atmosphere is substantially different in nature from that calculated for 1,000 aircraft with the 2050 atmosphere, in part because of specified changes in chlorine and methane contents of the stratosphere (see Chapter 4). Nevertheless, the best RF values are about the same: -0.01 W m-2. The uncertainty range in these values is large and changes sign (-0.04 to +0.01 W m-2), reflecting not only the range in O3 perturbations given by Chapter 4 but also the large uncertainty in deriving RF for stratospheric perturbations. This ozone-related RF for the HSCT fleet is based only on the stratospheric ozone perturbation calculated by the models in Chapter 4; the tropospheric changes are discarded, and a correction for the displacement of about 11% of the subsonic traffic is included, as shown in Table 6-1 (scenario Fa1H).

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